Plasma chambers commonly are used to perform processes for fabricating electronic devices such as semiconductors, displays and solar cells. Such plasma fabrication processes include chemical vapor deposition of semiconductor, conductor or dielectric layers on the surface of a workpiece or etching of selected portions of such layers on the workpiece surface.
A plasma commonly is sustained in a plasma chamber by coupling RF power from an RF applicator to a gas or plasma within the chamber. The RF power excites the gas to a plasma state or provides the RF power necessary to sustain the plasma. Two broad categories of coupling techniques are an electrode that capacitively couples RF power to the plasma or an antenna that radiates electromagnetic radiation into the plasma.
One conventional type of antenna is an inductive coupler, also called an inductively coupled antenna, in which RF power is coupled to the plasma primarily by means of the magnetic field produced by the antenna. A shortcoming of an inductive coupler is that it generally cannot be operated at an RF frequency whose wavelength is less than the diameter of the inductive coupler. The inability to operate at a high RF frequency is a serious shortcoming in certain plasma chemistries.
Another conventional type of antenna is a hollow waveguide having slots in one waveguide wall through which RF power is radiated from the interior of the hollow waveguide to the plasma. A shortcoming of a hollow waveguide is that it cannot operate below a cutoff frequency, hence its width along one transverse axis must be at least one-half the wavelength of a signal propagating within the waveguide at the power source frequency. As a result of this width requirement, slotted hollow waveguide antennas typically have been used outside a dielectric window of a plasma chamber rather than inside a plasma chamber.
Another conventional type of antenna is a linear conductor surrounded by a cylindrical dielectric, with the combination being positioned within a plasma chamber so that it is surrounded by the plasma. One or both ends of the conductor are connected to receive power from a UHF or microwave power source. Power is coupled from the antenna to the plasma by means of an electromagnetic wave at the boundary between the plasma and the dielectric. A shortcoming of this type of antenna is that the power radiated by the antenna progressively decreases with distance from the end of the antenna that is connected to the power source. Even if both ends of the antenna are connected to a power source, the radiated power near the center of the antenna will be lower than the power near the ends, thereby degrading spatial uniformity of the plasma. The non-uniformity increases with the length of the antenna, hence this type of antenna is less desirable for large plasma chambers.